REFRACTION THROUGH PRISMS. 4G5 



done, so that the air may escape easily. The aperture need not be 

 closed afterwards, as the water is kept in by the external pressure 

 of the atmosphere ; when the prism is to be emptied, the water may 

 be removed by sucking, using the same tube which served for 

 filling the prism. Immediately after using the prism, it is advisable 

 to remove the water as completely as possible, otherwise it pene- 

 trates into the wood, which becomes warped in consequence. If it 

 should become necessary to remove the glass plates in order to clean 

 the inner sides, the plates must be loosened by heating them with 

 the greatest caution, as the adhering moisture renders them liable 

 to crack when being heated. The wood should be thoroughly dried 

 before fixing the glass plates again upon it. 



For the construction of a second prism a large sound cork, a so- 

 called 'bung-cork,' may be used. Cut from it a round disk, 3 to 5 cm 

 in diameter and 15 to 20 mm thick, which may be employed like the 

 wedge just described. The faces are cut even and filed so that 

 they may slant; a hole is bored through the middle and widened, 

 leaving only a ring of cork as shown in fig. 263 B. The thickness of 

 the ring at the top should be 5 mm , and from 15 to 20 mm at the bottom. 

 Opposite to the thinnest portion a hole is bored for the handle s, which 

 is fixed with sealing-wax ; the hole a for filling the prism must in 

 this case be burned with a wire. The glass plates are of course 

 circular in this prism. 



Best window-glass, or still better thin plate-glass, should be used. 

 Better kinds of prisms for containing liquids may be made entirely 

 of glass ; their construction is described in the next article. 



Two prisms having different refracting angles will be required for 

 showing the influence of the magnitude of this angle upon the 

 deviation. 



Let, in fig. 265, b represent a small prism of glass 

 which refracts a ray of light proceeding from a in such 

 a manner that it reaches the point c. The prism e?, 

 having a smaller refracting angle than b, produces less 

 deviation, and if the refracting angle be suitably adjusted 

 the ray a d may also be refracted towards c ; similarly, 

 if the prism e has a still smaller refracting angle, and 

 the latter is properly chosen, the ray a e will meet the 

 other rays also at c. These three prisms have their 

 refracting edges directed upwards ; three corresponding 



H H 





